Device for activating trailer electric wheel brakes

ABSTRACT

An electric trailer brake controller includes a deceleration sensor for generating a brake control signal. The controller also includes a device to decrease the sensitivity of the deceleration sensor to spurious inputs. Additionally, the controller includes a brake current limiting circuit that progressively reduces the current supplied to the controlled trailer brake when the brake current exceeds a predetermined level.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation-in-Part of U.S. patentapplication Ser. No. 09/302,813, filed on Apr. 30, 1999.

BACKGROUND OF THE INVENTION

[0002] This invention relates in general to devices for actuatingtrailer electric wheel brakes and in particular to enhancements fortrailer electronic wheel brake controllers.

[0003] Towed vehicles, such as recreational and utility trailers whichare towed by automobiles and small trucks, are commonly provided withelectric wheel brakes. The electric wheel brakes generally include apair of brake shoes which, when actuated, frictionally engage a brakedrum. An electromagnet is mounted on one end of a lever to actuate thebrake shoes. When an electric current is applied to the electromagnet,the electromagnet is drawn against the rotating brake drum which pivotsthe lever to actuate the brakes. Typically, the braking force producedby the brake shoes is proportional to the electric current applied tothe electromagnet. This electric current can be relatively large. Forexample, the electric wheel brakes on a two wheeled trailer can draw sixamperes of current when actuated and the electric wheel brakes on a fourwheeled trailer can draw 12 amperes of current.

[0004] Automotive industry standards require that electrically-actuatedvehicle wheel brakes be driven against the ground potential of thevehicle power supply. Accordingly, one end of each of the towed vehiclewheel brake electromagnets is electrically connected to the towedvehicle ground and the towed vehicle ground is electrically connected tothe towing vehicle ground. The other end of each of the brakeelectromagnets is electrically connected through either an electricwheel brake actuator or an electric wheel brake controller to the towingvehicle power supply.

[0005] Generally, electric wheel brake actuators are manually operateddevices which control the magnitude of electric current supplied to thetowed vehicle wheel brakes. Various electric brake controllers for towedvehicle electric brakes are known in the art. For example, a variableresistor, such as a rheostat, can be connected between the towingvehicle power supply and the brake electromagnets. Such an actuator isdisclosed in U.S. Pat. No. 3,740,691. The towing vehicle operatormanually adjusts the variable resistor setting to vary the amount ofcurrent supplied to the brake electromagnets and thereby control theamount of braking force developed by the towed vehicle wheel brakes.

[0006] It is also known to include an integrating circuit in an electricwheel brake actuator. When the towing vehicle brakes are applied, asignal is sent to the integrating circuit. The integrating circuitgenerates a continually increasing voltage which is applied to theelectric wheel brakes. The longer the towing vehicle brakes are applied,the more brake torque is generated by the actuator. A manuallyadjustable resistor typically controls the rate of integration. On suchactuator is disclosed in U.S. Pat. No. 3,738,710.

[0007] Also known in the art are more sophisticated electric wheel brakecontrollers which include electronic circuitry to automatically supplycurrent to the towed vehicle brake electromagnets which is proportionalto the towing vehicle deceleration when the towing vehicle brakes areapplied. Such electronic wheel brake controllers typically include asensing unit which generates a brake control signal corresponding to thedesired braking effort. For example, the sensing unit can include apendulum which is displaced from a rest position when the towing vehicledecelerates and an electronic circuit which generates a brake controlsignal which is proportional to the pendulum displacement. One such unitis disclosed in U.S. Pat. No. 4,721,344. Alternately, the hydraulicpressure in the towing vehicle's braking system or the pressure appliedby the vehicle operator's foot to the towing vehicle's brake pedal canbe sensed to generate the brake control signal. An example of acontroller which senses the towing vehicle brake pressure to generatethe brake control signal is disclosed in U.S. Pat. No. 4,398.252.

[0008] Known electronic wheel brake controllers also usually include ananalog pulse width modulator. The input of the pulse width modulator iselectrically connected to the sensing unit and receives the brakecontrol signal therefrom. The pulse width modulator is responsive to thebrake control signal for generating an output signal comprising a fixedfrequency pulse train. The pulse width modulator varies the duty cycleof the pulse train in direct proportion to the magnitude of the brakecontrol signal. Thus, the duty cycle of the pulse train corresponds tothe amount of braking effort desired.

[0009] Electronic wheel brake controllers further include an outputstage which is electrically connected to the output of the pulse widthmodulator. The output stage typically has one or more power transistorswhich are connected between the towing vehicle power supply and thetowed vehicle brake electromagnets. The power transistors, which areusually Field Effect Transistors (FET's), function as an electronicswitch for supplying electric current to the towed vehicle brakes. Theoutput stage may also include a drive circuit which electrically couplesthe output of the pulse width modulator to the gates of the FET's.

[0010] The output stage is responsive to the pulse width modulatoroutput signal to switch the power transistors between conducting, or“on”, and non-conducting, or “off”, states. As the output transistorsare switched between their on and off states in response to themodulator output signal, the brake current is divided into a series ofpulses. The power supplied to the towed vehicle brakes and the resultinglevel of brake application are directly proportional to the duty cycleof the modulator generated output signal.

SUMMARY OF THE INVENTION

[0011] This invention relates to enhancements for trailer electronicwheel brake controllers.

[0012] As explained above, electronic wheel brake controllers energizethe towed vehicle brakes upon detection of the towing vehicledeceleration when the towing vehicle brakes are applied. However, otherconditions, such as travel over a rough road surface may cause thependulum in a deceleration sensor to be displaced and thereby generate afalse brake control signal. Accordingly, it would be desirable to reducethe sensitivity of the sensing unit to filter out such spurious inputs.

[0013] Additionally, modem towing vehicles are equipped with capacityalternators that can supply large amounts of current. Furthermore, thevoltage output of such alternators tends to fluctuate with loadconditions. Accordingly, the current supplied to the trailer brakescould, under certain conditions become excessive. However, use of a fuseor circuit breaker to protect the trailer brake coils is not desirablesince it would have to be replaced or reset after every occurrence ofexcessive current. Accordingly, it would also be desirable to provide ameans for limiting the current supplied from the wheel brake controllerto the trailer brakes.

[0014] The present invention contemplates a device for controlling theelectric current supplied to at least one electric wheel brake whichincludes a brake control signal generator which is adapted to beconnected to a vehicle stop light switch. The brake control signalgenerator is operative to generate a brake control signal that is afunction of the towing vehicle deceleration. The device also includes abrake control signal amplifier that is connected to the brake controlsignal generator. A damping capacitor is coupled to the brake controlsignal amplifier to reduce sensitivity of the brake control signalgenerator. The brake control signal amplifier has an output that isconnected to an output signal generator. The output signal generator hasan output terminal and is responsive to an amplified brake controlsignal to generate an output signal at the output terminal which is afunction of the brake control signal. The device further includes anelectric current controller which is adapted to be connected between avehicle power supply and the controlled electric wheel brake. Thecurrent controller also is coupled to the output terminal of the outputsignal generator and is responsive to the output signal to control theelectric current supplied to the controlled wheel brake as a function ofthe output signal.

[0015] The invention further contemplates a current limiting circuitcoupled to the current controller and the output signal generator. Thecurrent limiting circuit is operable to modify the output signal toprogressively reduce the current supplied to the controlled wheel brakeupon the current exceeding a first predetermined threshold.Additionally, the current limiting circuit is operative to disable theoutput signal generator upon the current being supplied to thecontrolled wheel brake exceeding a second predetermined threshold whichis greater than the first predetermined threshold.

[0016] Various objects and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the preferred embodiment, when read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic circuit diagram for an electric brakeactuator in accordance with the invention.

[0018]FIG. 2 is a graph of selected voltages within the actuator shownin FIG. 1 during a brake application.

[0019]FIG. 3 is a graph of selected voltages within the actuator shownin FIG. 1 during operation of the towing vehicle hazard flasher.

[0020]FIG. 4 is a schematic circuit diagram for an electric brakecontroller in accordance with the invention.

[0021]FIG. 5 is a schematic circuit diagram for an alternate embodimentof the electric brake controller shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Referring now to the drawings, there is illustrated in FIG. 1 aschematic circuit diagram for an enhanced electric brake actuator 10.The actuator 10 includes an input operational amplifier U1 c whichgenerates a brake control signal at its output terminal when the towingvehicle brakes are applied. The input operational amplifier U1 c has apositive input terminal which is connected through an input resistor R26to a towing vehicle stop light switch 15. A ramp capacitor C4, thepurpose for which will be explained below, is connected between thepositive input terminal of the operational amplifier U1 c and ground.

[0023] The actuator 10 further includes first and second operationalamplifiers, which are identified by the designators U1 a and U1 b,respectively. The output terminal of the input operational amplifier U1c is connected to a positive input terminal of the first operationalamplifier U1 a. Thus, the brake control signal is applied to thepositive input terminal of the first operational amplifier U1 a. Thefirst operational amplifier U1 a also has a negative input terminalwhich is connected to an output terminal of the second operationalamplifier U1 b. The first and second operational amplifiers U1 a and U1b are responsive to the brake control signal exceeding a thresholdvoltage to generate a PWM output signal at an output terminal of thefirst operational amplifier U1 a. In the preferred embodiment, thethreshold voltage is approximately two volts.

[0024] The PWM output signal has a duty cycle which is proportional tothe magnitude of the brake control signal.

[0025] The output terminal of the first operational amplifier U1 a isconnected to the base of a driver transistor Q4. The collector of thedriver transistor Q4 is connected to the gate of an output power FieldEffect Transistor (FET) Q1. The power FET Q1 is connected between thetowing vehicle power supply 16 and the towed vehicle electric brakecoils 18 (one shown). An actuation transistor Q5 is connected betweenthe emitter of the driver transistor Q4 and ground. The actuationtransistor Q5 has a base terminal connected through an actuation Zenerdiode D3 and a series connected pair of resistors, R19 and R35, to thetowing vehicle stop light switch 15. Closure of the stop light switch 15upon application of the towing vehicle brakes causes the actuationtransistor Q5 to be in a conducting state and thus enables the controlof the output FET Q1 by the driver transistor Q4.

[0026] When the stop light switch 15 is closed, the ramp capacitor C4charges through the input resistor R26 with a time constant which is afunction of the product of the ramp capacitor C4 and the input resistorR26. Accordingly, as the stop light switch 15 remains closed, anincreasing voltage is applied to the positive input terminal of theinput operational amplifier U1 c. In response to the increasing voltage,the operational amplifier U1 c generates an increasing ramped brakecontrol signal which is applied to the positive input terminal of thefirst operational amplifier U1 a. The first and second operationalamplifiers U1 a and U1 b co-operate to generate a PWM output signalhaving a constant frequency and a ramped duty cycle which isproportional to the magnitude of the brake control signal. The PWMoutput signal is applied to the base of the driver transistor Q4.

[0027] The driver transistor Q4 is responsive to the output signal toswitch the power FET Q1 between its non-conducting and conducting stateswith the duration of the conducting states increasing as the rampcapacitor C4 charges. As the power FET Q1 remains in its conductingstate for a longer portion of each switching cycle, the average currentsupplied to the brake coils 18 increases. Thus, the magnitude of thecurrent supplied to the brake coils 18 increases as a function of thetime constant determined by the product of the input resistor R26 andramp capacitor C4.

[0028] The towing vehicle also includes a hazard flasher switch 19,which is connected in parallel across the stop light switch 15. Asexplained above, the operation of the towing vehicle's hazard flasherswitch 19 can cause false actuation of the towed vehicle brakes.Accordingly, the present invention contemplates including a hazard delayand automatic reset circuit, which is shown in FIG. 1 within the dashedlines labeled 20, in the actuator circuit 10. The hazard delay circuit20 includes a delay capacitor C3 which has a first end connected to acenter tap of a first voltage divider 21 and a second end connected toground. The first end of the delay capacitor C3 also is connectedthrough a coupling diode D6 to the positive input terminal of the firstoperational amplifier U1 a. The first voltage divider 21, which includesa pair of resistors, R27 and R28, is connected between the collector ofan isolation transistor Q7 and ground. The isolation transistor Q7 hasan emitter connected through a plurality of diodes, D13, D14 and D15, tothe stop light switch 15. As will be explained below, during normaloperation of the actuator 10, the isolation transistor Q7 is in itsconducting state.

[0029] The hazard delay circuit 20 also includes a second voltagedivider 22, which includes a pair of resistors, R29 and R30, connectedbetween the collector of the isolation transistor Q7 and ground. Thecenter tap of the second voltage divider 22 is connected to the base ofa discharge transistor Q6. Thus, the second voltage divider 22 functionsto bias the discharge transistor Q6. The emitter of the dischargetransistor Q6 is connected through a first discharge diode D11 to thecenter tap of first voltage divider 21 and thereby to the non-groundedfirst end of the delay capacitor C3. The emitter of the dischargetransistor Q6 also is connected through a second discharge diode D7 tothe positive input terminal of the input operational amplifier U1 c andthereby to the non-grounded side of the ramp capacitor C4.

[0030] The operation of the hazard delay and automatic reset circuit 20will now be explained. Selected voltages within the actuator 10 during anormal brake actuation, without the hazard flasher in operation, areillustrated in FIG. 2. Before actuation of the stop light switch 15,both the delay capacitor C3 and the ramp capacitor C4 are discharged.Also, the base of the discharge transistor Q6 is at ground potential,which causes the discharge transistor Q6 to be in its conducting state.Accordingly, when the stop light switch 15 is closed, as shown at t₁ inthe top curve in FIG. 1, the power supply voltage is applied to thesecond voltage divider 22. A portion of the power supply voltage appearson the base of the discharge transistor Q6 which causes the transistorQ6 to switch to its non-conducting state, blocking current flow throughthe first and second discharge diodes D11 and D7. The delay capacitor C3proceeds to charge through the resistor R27 to a voltage leveldetermined by the ratio of the resistors in the first voltage divider21, as shown in the curve labeled “C3” in FIG. 2. Simultaneously withthe charging of the delay capacitor C3, the ramp capacitor C4 chargesthrough the input resistor R26 causing the input operational amplifierU1 c to generate a ramped brake control signal, as shown by the curvelabeled “RAMP” in FIG. 2. Both the voltage across the delay capacitor C3and the ramped brake control signal, RAMP, generated by the inputoperational amplifier U1 c are applied to the positive input terminal ofthe first operational amplifier U1 a. As can be seen in FIG. 2,initially, the voltage across C3 increases at a faster rate that thebrake control signal, RAMP. Accordingly, the first operational amplifieris initially responsive to the voltage across the delay capacitor C3.When the voltage across the delay capacitor C3 increases to thepredetermined threshold level, which occurs at t₂ in FIG. 2, the firstoperational amplifier U1 a begins to generate the PWM output signalwhich causes actuation of the towed vehicle brakes.

[0031] In the preferred embodiment, the curve labeled C3 initiates a PWMoutput signal having a duty cycle of 8 to 12 percent, as illustrated inthe bottom curve in FIG. 2. The reduced duty cycle provides a “softturn-on” for the towed vehicle brakes. At t₃, the ramp brake controlsignal generated by the input operational amplifier U1 c exceeds thevoltage across the delay capacitor C3 and causes the duty cycle of thePWM output signal to ramp up to a maximum of 100 percent, which isreached at t₄. The duty cycle remains at 100 percent until the stoplight switch 15 is released at t₅. The slope of the ramp brake controlsignal, RAMP, generated by the input operational amplifier U1 c isadjustable with the Automatic Gain Control (AGC), R8. Thus, under normaloperating conditions, the delay capacitor C3 and ramp capacitor C4function to slightly delay the application of and provide a soft turn-onto the towed vehicle brakes

[0032] Selected voltages within the actuator 10 with the hazard flasheractuated are shown in FIG. 3. When the hazard flasher of the towingvehicle is actuated, the hazard flasher switch 19 is periodically movedbetween open and closed positions. Thus, the hazard flasher switch 19closes at t₆ and opens at t₇ in FIG. 3. Accordingly, the input voltageto the actuator 10 consists of a pulse train, as illustrated by the stoplight voltage curve shown at the top of FIG. 3. The time constant forthe delay RC circuit comprising R27 and C3 is selected such that thedifference between t₁ and t₂ is slightly greater than the on-time of thetowing vehicle hazard flasher switch 19, which is the difference betweent₆ and t₇. In the preferred embodiment, the time constant provides adifference between t₁ and t₂ which is approximately a half second. Theramp RC circuit comprising R26 and C4 has a time constant which islonger than the delay RC time constant. Accordingly, if the inputvoltage to the actuator 10 is generated by the hazard flasher, the inputvoltage to the actuator 10 will go to zero before the delay capacitor C3charges sufficiently to initiate generation of a PWM output signal, asshown in the middle and lower curves in FIG. 3.

[0033] When the actuator input voltage returns to zero, the base of thedischarge transistor Q6 is pulled to ground, causing the dischargetransistor Q6 to switch to its conducting state. When the dischargetransistor Q6 begins to conduct, the delay capacitor C3 beginsdischarging through the first discharge diode D11 and the ramp capacitorC4 begins discharging through the second discharge diode D7 to preparethe circuit 20 for the next on-cycle of the hazard flasher. It will beappreciated that the discharge transistor Q6 and discharge diodes D11and D7 also begin to conduct to discharge the delay and ramp capacitorsC3 and C4 upon the stop light switch 15 opening at the end of a normalbraking cycle.

[0034] As explained above, the actuator 10 includes a manual brakecontrol which can be used by the towing vehicle operator to apply thetrailer brakes independently of the towing vehicle brakes. The manualbrake control includes a potentiometer R7 which is connected between thetowing vehicle power supply 16 and ground. The slider tap of thepotentiometer R7 is connected to the positive input terminal of thefirst operational amplifier U1 a. Movement of the potentiometer R7 fromits “OFF” position generates a manual brake control signal which isapplied to the first operational amplifier U1 a. However, if theautomatic gain control of the input operational amplifier U1 c is settoo high, an application of the towing vehicle brakes could cause theinput operational amplifier U1 c to generate a greater than needed brakecontrol signal. Accordingly, the present invention further contemplatesthat the actuator 10 includes a manual stop light and automaticisolation circuit, which is labeled 30 in FIG. 1.

[0035] As shown in FIG. 1, the manual brake control signal potentiometerR7 is ganged to a manual control potentiometer switch S1. In thepreferred embodiment, the potentiometer R7 includes a return springwhich urges the potentiometer slider to the OFF position. When thetowing vehicle operator manually moves the slider from the OFF position,the switch S1 is closed. One side of the switch S1 is connected to thevehicle power supply 16. The normally open contact of the switch S1 isconnected through the coil of a relay RE1 to ground. The relay RE1includes a set of normally open contacts connected between the powersupply 16 and the stop light lamp. The normally open contact of theswitch S1 is connected to the base of the isolation transistor Q7, thesecond operational amplifier U1 b and the vehicle stop lights (oneshown).

[0036] The operation of the manual stop light and automatic isolationcircuit 30 will now be described. During normal operation, the switch S1is open, causing the base of the isolation transistor Q7 to be at groundpotential. Accordingly, the isolation transistor Q7 is normally in itsconducting state which allows power to flow from the stop light switch15 to the delay and ramp capacitors, C3 and C4. However, upon movementof the slider of the manual brake control signal potentiometer R7 togenerate a manual brake control signal, the switch S1 is closed. Whenthe switch S1 closes, a voltage is applied to the base of the isolationtransistor Q7 which causes the transistor to switch to itsnon-conducting state. Also, the relay contacts close to illuminate thestop light lamp. With the isolation transistor Q7 in a non-conductingstate, the delay and ramp capacitors, C3 and C4, are isolated from thestop light switch 15. Accordingly, actuation of the stop light switch 15when the manual control is in use will not cause the input operationalamplifier U1 c to generate a brake control signal. As described above,closure of switch S1 supplies power to the second operational amplifierU1 b which enables the generation of a PWM output signal from the firstoperational amplifier U1 a in response to the manual brake controlsignal. As described above, power also is supplied to illuminate thetowing and towed vehicle stop lights (one shown).

[0037] The actuator 10 also includes an output current limiting andshort circuit protection circuit 40. The circuit 40 includes a currentsensor 41 comprising a plurality of low valued resistors which areconnected in parallel. In the preferred embodiment, three 0.10 ohmresistors, which are labeled R11, R12 and R13 in FIG. 1, are connectedin parallel; however, more or less resistors can be utilized. Thecurrent sensor 41 is connected between the power supply 16 and thesource terminal of the output power FET Q1. As described above, thepower output FET Q1 has a drain terminal connected through the coils 18(one shown) of the electric wheel brakes to ground. The end of thecurrent sensor 41 connected to the source terminal of the FET Q1 isconnected thorough a resistor R16 to a base terminal of a first sensortransistor Q2. The first sensor transistor Q2 has an emitter terminalconnected to the power supply 16 and a collector terminal connectedthrough a sensor capacitor C2 to ground.

[0038] The collector terminal of the first sensor transistor Q2 also isconnected to a bias circuit 42 comprising a pair of resistors, labeledR17 and R33, connected in series. The center tap of the bias circuit 42is connected to the base of a second sensor transistor Q3. The emitterof the second sensor transistor Q3 is connected to ground while thecollector of the second sensor transistor Q3 is connected through ablocking diode D8 to the positive input terminal of a first operationalamplifier U1 a. The blocking diode D8 blocks current from flowing backto the first operational amplifier input terminal from the currentlimiting circuit 40.

[0039] The operation of the current limiting circuit 40 will now bedescribed. When the output FET Q1 conducts, a load current flows throughthe current sensor 41. The load current causes a voltage to appearacross the current sensor 41 which is directly proportional to themagnitude of the load current. When the voltage across the currentsensor 41 exceeds a first predetermined threshold, the first transistorQ2 begins to conduct which causes the sensor capacitor C2 to begin tocharge. It will be appreciated that the load current flowing through theoutput FET Q1 fluctuates as the PWM output voltage switches the FET Q1between its conducting and non-conducting states. Accordingly, thecurrent flowing to the sensor capacitor C2 also fluctuates. The sensorcapacitor C2 smoothes the fluctuations and charges to a voltage which isproportional to the average load current supplied to the brake coils 18.The voltage across the sensor capacitor C2 is applied to the base of thesecond sensor transistor Q3. which turns on and thereby reduces thebrake control signal applied to the positive input terminal of the firstoperational amplifier U1 a. The reduced brake control signal causes, inturn, a reduction in the duty cycle of the PWM output voltage. Thereduced duty cycle reduces the on time of the output FET Q1 and,thereby, reduces the load current supplied to the electric trailer brakecoils 18.

[0040] If the current supplied to the trailer brake coils 18 furtherincreases, the voltage across the current sensor 41 also increases,progressively turning on the first and second sensor transistors Q2 andQ3 and thereby progressively reducing the duty cycle of the PWM outputvoltage. Upon the load current reaching a second predeterminedthreshold, the second transistor Q3 becomes fully conducting, providinga direct connection between the positive input terminal of the firstoperational amplifier U1 a and ground. When this occurs, the brakecontrol signal is shunted to ground and the operational amplifier PWMoutput signal goes to zero, turning off the output FET Q1 and providingshort circuit protection for the actuator 10. In the preferredembodiment, the sensor transistors Q2 and Q3 in the current limitingcircuit 40 begin conducting when the brake current reaches 13.5 to 18amps and complete shut off of the output FET Q1 occurs when the outputcurrent reaches approximately 20 to 24 amps. The current values can beadjusted by selecting other values for the sensor capacitor C2 and/orthe resistors R17 and R33.

[0041] Upon shut off of the output FET Q1, the first sensor transistorQ2 also is shut off as the current flow though the current sensor 41stops. The sensor capacitor C2 then begins to discharge through the biasresistors R17 and R33. As the sensor capacitor C2 discharges, theconduction of the second sensor transistor Q3 is progressively reduced,allowing the voltage at the positive input terminal to the firstoperational amplifier U1 a to increase. In the preferred embodiment, thetime constant for the combination of the sensor capacitor C2 and theresistors R17 and R33 is selected such that, for brake currents inexcess of 20 amps, the sensor capacitor C2 will maintain a sufficientlyhigh charge to keep the brake current at zero for three cycles of thePWM signal. Thus, the actuator off-time is increased to approximately 11milliseconds from a typical off-time of approximately 3 milliseconds inprior art actuators. As a result, the heating of the power FET Q1 isgreatly reduced. The invention also contemplates using power FET'shaving a lower internal resistance than in prior art controllers tofurther reduce heating and associated power losses.

[0042] The invention further contemplates that the brake actuator 10includes a plurality of the voltage regulation diodes labeled D13, D14and D15 which are connected between the stop light switch 15 and thepositive input terminal of the input operational amplifier U1 c. Theregulation diodes D13 thorough D15 reduce the input voltage supplied tothe actuator 10 from the stop light switch and compensate for variationof the towing vehicle alternator voltage. When conducting, the voltageacross each of the regulation diodes is fixed by the diode forward emfand does not vary with the supplied voltage as the voltage across aresistive voltage divider would. While three regulation diodes are shownin FIG. 1, it will be appreciated that the invention also can bepracticed with more or less diodes.

[0043] The invention further contemplates stabilizing the voltageswithin the actuator and controller circuit 10 with selected use of onepercent tolerance resistors. Such resistors do not vary with temperaturechanges or the age of components. In the preferred embodiment, onepercent resistors are utilized for the resistors R27 and R28 in thefirst voltage divider 21 to assure that the actuator 10 has a consistentturn on duty cycle for the PWM output signal.

[0044] The invention also contemplates utilizing the output limiter andshort circuit protection circuit 40 in an enhanced electric brakecontroller 50, as illustrated in FIG. 4. Components shown in FIG. 4which are similar to components shown in FIG. 1 have the same numericaldesignators. The electric brake controller 50 is similar to the actuator10, but includes a brake control signal generator 55. In the preferredembodiment shown in FIG. 4, the brake control signal generator 55includes a pendulum device (not shown) which co-operates with a HallEffect Device (HED) 56 to generate a brake control signal which isproportional to the deceleration of the towing vehicle. The brakecontrol signal is applied to the positive input terminal of the firstoperational amplifier U1 a. As described above, the first operationalamplifier U1 a cooperates with a second operational amplifier U1 b togenerate a PWM output signal for controlling the output power FET Q1.The PWM output signal has a duty cycle which is a function of the brakecontrol signal.

[0045] As shown in FIG. 4, the enhanced controller 50 includes theoutput limiter and short circuit protection circuit 40 described above.The protection circuit 40 monitors the current flowing through theoutput FET Q1 and is operable to reduce the duty cycle of the PWM outputsignal as the current increases above a predetermined first threshold.The protection circuit 40 is further operable to turn off the output FETQ1 if the current exceeds a second predetermined threshold. Similar tothe actuator 10 described above, the controller off-time is increased toapproximately 11 milliseconds from a typical off-time of approximately 3milliseconds in prior art cintrollers.

[0046] The present invention contemplates use of zener diodes toregulate voltages in the brake controller circuit 50 shown in FIG. 4. Afirst zener diode, which is labeled D4, is connected between the voltageinput terminal of the second operational amplifier U1 b and ground. Thefirst zener diode D4 functions to regulate the voltage supplied to theoperational amplifier and thus prevent overloading the operationalamplifier while assuring consistent operation of thereof. A second zenerdiode, which is labeled D7, is connected between the voltage inputterminal of the HED 56 and ground. The second zener diode D7 functionsto regulate the voltage supplied to the HED 56 and thus preventoverloading the HED 56 while assuring generation of consistent automaticbrake control signals. A third zener diode, which is labeled D10, isconnected between the voltage input terminal of the manual brake controlsignal potentiometer P2 and ground. The third zener diode D10 functionsto regulate the voltage supplied to the potentiometer P2 and thusprevent overloading the potentiometer P2 while assuring generation ofconsistent manual brake control signals. A fourth zener diode D3 isconnected between the stop light switch 15 and the base of the actuationtransistor Q5. The fourth zener diode D3 provides a threshold voltagewhich must be exceed before the output power FET Q1 can be turned on. Itwill be noted that the fourth zener diode D3 also is included in theimproved actuator circuit 10 shown in FIG. 1. Additionally, the zenerdiodes, D3, D4, D7 and D10 are selected to have a positive temperaturecoefficient to prevent a temperature increase from decreasing the dutycycle of the PWM output signal.

[0047] The invention also contemplates the inclusion of a dampingcapacitor C13 which is connected between the output terminal and thenegative input terminal of the input operational amplifier U1 c. Thedamping capacitor C13 slows changes in the automatic brake controlsignal to prevent false brake applications caused by road surfaceirregularities displacing the pendulum device. In the preferredembodiment, the damping capacitor C13 is a 1.0 micro-farad capacitor.Damping can be further increased by connecting an optional seconddamping capacitor C14 in parallel to the damping capacitor C13, as shownin FIG. 4.

[0048] The controller 50 further includes a voltage divider 57 whichsupplies a minimum brake control signal to the positive input terminalof the first operational amplifier U1 a. The voltage divider 57 includesa pair of resistors R27 and R28 which are connected between the stoplight switch 15 and ground. When the stop light switch 15 is closed, asmall voltage is applied to the positive input terminal of the firstoperational amplifier U1 a to actuate the trailer wheel brakes beforethe towing vehicle has decelerated sufficiently for the pendulum device55 and HED 56 to generate an automatic brake control signal. In thepreferred embodiment, the minimum brake control signal is equivalent toa ten percent brake application; however, by adjusting the values of theresistors R27 and R28, other amounts of brake application can beprovided, such as a six percent initial application. Also in thepreferred embodiment, one percent resistors are utilized for theresistors R27 and R28 in the voltage divider 57 to assure that thecontroller 50 has a consistent turn on duty cycle for the PWM outputsignal.

[0049] An alternate embodiment 60 of the circuit 50 is shown in FIG. 5.Components in FIG. 5 that are similar to components shown in FIG. 4 havethe same numerical designators. The circuit 60 includes a voltagestabilizing circuit 62 that replaces three of the voltage regulatingcircuits included in the circuit 50 shown in FIG. 4. The voltagestabilizing circuit 62 includes a series connection of a resistor R31with a cathode of a Zener diode D7. The anode of the Zener diode D7 isconnected to ground while the end of the resistor R31 that is oppositefrom the Zener diode D7 is connected to the power supply 16 througheither the stop light switch 15 or the relay RE1. A regulated voltagesupply appears at the cathode of the Zener diode D7. The cathode of theZener diode D7 is connected to the voltage input terminal of the secondoperational amplifier U1 b, the voltage input terminal of the HED 56 andthe voltage input terminal of the manual brake control signalpotentiometer P2. Accordingly, two of the Zener Diodes, D4 and D10, thatare included in the circuit 50 shown in FIG. 4 are eliminated. This notonly reduces the cost of the circuit 60, but also eliminates variationin the regulated voltage supplied to the components due to the tolerancedifferences from use of three Zeners.

[0050] Additionally, in the circuit 60, a minimum turn on potentiometerP4 is connected between the manual control signal potentiometer P2 andground. The minimum turn on potentiometer P4 provides an initial inputsignal to the positive input terminal of the first operational amplifierU1 a. Thus, the potentiometer P4 replaces the voltage divider 57 shownin FIG. 4 and provides an adjustable initial voltage.

[0051] In accordance with the provisions of the patent statutes, theprinciple and mode of operation of this invention have been explainedand illustrated in its preferred embodiment. However, it must beunderstood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope. For example, the isolation circuit 30 included in the actuator10 shown in FIG. 1 also can be included in the brake controller 50illustrated in FIG. 4.

What is claimed is:
 1. A controller for controlling the electric currentsupplied to at least one electric wheel brake comprising: a brakecontrol signal generator which is adapted to be connected to a vehiclestop light switch, said brake control signal generator including adevice for sensing deceleration of a towing vehicle and generating abrake control signal which is a function of the towing vehicledeceleration; a brake control signal amplifier having an input terminaland an output terminal, said brake control signal amplifier inputterminal connected to said brake control signal generator, saidamplifier operative to amplify said brake control signal; at least onedamping capacitor connected to said brake control signal amplifier, saiddamping capacitor operative to decrease the sensitivity of saiddeceleration sensor; an output signal generator having an input terminalwhich is connected to said output terminal of said brake control signalamplifier, said output signal generator also having an output terminal,said output signal generator being responsive to said amplified brakecontrol signal to generate an output signal at said output terminalwhich is a function of said amplified brake control signal; and anelectric current controller which is adapted to be connected between avehicle power supply and the controlled electric wheel brake, saidelectric current controller coupled to said output terminal of saidoutput signal generator, said electric current controller responsive tosaid output signal to supply an electric current to the controlled wheelbrake which is a function of said output signal.
 2. A controlleraccording to claim 1 also including a voltage divider connected betweensaid vehicle stop light switch and ground, said voltage divider coupledto said output signal generator, said voltage divider operative uponclosure of said stop light switch to cause said output signal generatorto generate a minimum output signal.
 3. A controller according to claim2 wherein said voltage divider includes resistors having toleranceswhich are less than ten percent whereby said output signal generator hasa consistent turn-on characteristic.
 4. A controller according to claim1 also including a potentiometer connected between said vehicle stoplight switch and ground, said potentiometer including a variable tapthat is coupled to said output signal generator, said potentiometeroperative upon closure of said stop light switch to apply an adjustablevoltage to said output signal generator to cause said output signalgenerator to generate a minimum output signal.
 5. A controller accordingto claim 1 further including at least one zener diode to control avoltage level within the controller.
 6. A controller according to claim4 wherein said zener diode has a positive temperature coefficientwhereby the controller output current does not decrease when thetemperature of said zener diode increases.
 7. A controller forcontrolling the electric current supplied to at least one electric wheelbrake comprising: a brake control signal generator which is adapted tobe connected to a vehicle stop light switch, said brake control signalgenerator including a device for sensing deceleration of a towingvehicle and generating a brake control signal which is a function of thetowing vehicle deceleration; an output signal generator having an inputterminal which is coupled to said brake control signal generator, saidoutput signal generator having an output terminal, said output signalgenerator responsive to said brake control signal to generate an outputsignal at said output terminal which is a function of said brake controlsignal; an electric current controller which is adapted to be connectedbetween a vehicle power supply and the controlled electric wheel brake,said electric current controller coupled to said output terminal of saidoutput signal generator, said electric current controller responsive tosaid output signal to control said electric current as a function ofsaid output signal; and an output current limiting circuit coupled tosaid electric current controller and said output signal generator, saidcurrent limiting circuit operable to modify said output signal toprogressively reduce said current supplied to said controlled wheelbrake upon said current exceeding a first predetermined threshold.
 8. Acontroller according to claim 7 wherein said current limiting circuitincludes at least one current sensing resistor connected between saidvehicle power supply and said current controller, said current sensingresistor coupled to a first sensor transistor, a sensing capacitorconnected between said first sensing transistor and ground, said sensingcapacitor coupled to a second sensing transistor, said second sensingtransistor coupled to said output signal generator, said first sensortransistor being progressively turned on as the current flowing thoughsaid sensing resistor increases above a first predetermined threshold,said sensing capacitor charging as said first sensing transistorconducts, the voltage across said sensing capacitor causing said secondsensing transistor to turn on and progressively reduce said brakecontrol signal supplied to said output signal generator.
 9. A controlleraccording to claim 8 wherein said output current limiting circuit isoperative to disable said output signal generator upon said currentsupplied to said controlled wheel brake exceeding a second predeterminedthreshold, said second predetermined threshold being greater than saidfirst predetermined threshold.
 10. A controller according to claim 7also including a voltage divider connected between said vehicle stoplight switch and ground, said voltage divider coupled to said outputsignal generator, said voltage divider operative upon closure of saidstop light switch to cause said output signal generator to generate aminimum output signal.
 11. A controller according to claim 10 whereinsaid voltage divider includes resistors having tolerances which are lessthan ten percent whereby said output signal generator has a consistentturn-on characteristic.
 12. A controller according to claim 7 alsoincluding a potentiometer connected between said vehicle stop lightswitch and ground, said potentiometer including a variable tap that iscoupled to said output signal generator, said potentiometer operativeupon closure of said stop light switch to apply an adjustable voltage tosaid output signal generator to cause said output signal generator togenerate a minimum output signal.
 13. A controller according to claim 7further including at least one zener diode to control a voltage levelwithin the controller.
 14. A controller according to claim 13 whereinsaid zener diode has a positive temperature coefficient whereby thecontroller output current does not decrease when the temperature of saidzener diode increases.
 15. A controller for controlling the electriccurrent supplied to at least one electric wheel brake upon actuation oftowing vehicle brakes, said controller comprising: a brake controlsignal generator which is adapted to be connected to a vehicle stoplight switch, said brake control signal generator including a device forsensing deceleration of a towing vehicle and generating a brake controlsignal which is a function of the towing vehicle deceleration; an outputsignal generator having an input terminal which is coupled to said brakecontrol signal generator, said output signal generator having an outputterminal, said output signal generator being responsive to said brakecontrol signal to generate an output signal at said output terminalwhich is a function of said brake control signal; an initial brakeactuation detection device coupled to said output signal generator, saidinitial brake actuation detection device operative upon actuation of thetowing vehicle brakes to cause said output signal generator to generatea minimum output signal; and an electric current controller which isadapted to be connected between a vehicle power supply and thecontrolled electric wheel brake, said current controller coupled to saidoutput terminal of said output signal generator, said current controllerresponsive to said output signal to supply an electric current to thecontrolled wheel brake which is a function of said output signal.
 16. Acontroller according to claim 15 wherein said initial brake actuationdetection device includes a voltage divider connected between saidvehicle stop light switch and ground, said voltage divider coupled tosaid output signal generator, said voltage divider operative uponclosure of said stop light switch to cause said output signal generatorto generate a minimum output signal.
 17. A controller according to claim16 wherein said voltage divider includes resistors having toleranceswhich are less than ten percent whereby said output signal generator hasa consistent turn-on characteristic.
 18. A controller according to claim15 wherein said initial brake actuation detection device includes apotentiometer connected between said vehicle stop light switch andground, said potentiometer including a variable tap that is coupled tosaid output signal generator, said potentiometer operative upon closureof said stop light switch to apply an adjustable voltage to said outputsignal generator to cause said output signal generator to generate aminimum output signal.